How do you keep from venting and blowing up at a sibling?

FRAULEIN remember that all negative feelings (even if they are absolutely provoked by someone else's behavior) hurts the person who feels it. Here the important thing is to free yourself from these feelings to heal yourself. Have you tried meditation? Even if you can find 5 minutes in your day is worth the results. It is NOT easy and at the beginning you have to do practically force yourself to do it as "lip service", but if you are constant and do it every day you will begin to notice a change of attitude in yourself. You will feel at peace with yourself and that will begin to free you from lashing out. What you do is you sit comfortably, you pay attention to your breathing (breathe slow, rhythmically and deeply), visualize your sister and repeat these affirmations over and over again for the entire five minutes: "May you be at peace"; "May your heart be open"; "May you awake to the light of your own true nature"; May you be healed"; "May you be a source of healing for everyone that surrounds you" and then during the day, every time you feel the anger, tell to yourself "I am letting go of this anger" repeat it three times and shift your attention to other things. Do not allow yourself to wallow on the toxicity of the anger and resentment that only harms you.

FRAULEIN, has your sister done something specific that may have set you off? Does she participate? Are there past issues? If she's done someting wrong, try to talk to her calmly about it. If she's just your target from your being tired, frustratated and burned out, tell her how you're feeling without lashing out. Just keep thinking in your mind how you'd like to be treated if she were the one angry at you for whatever the reason.
I handled my pent up anger totally wrong with my only sib. She lived 3000 mi away and even if she had wanted to participate, which she did NOT and told me many times, she couldn't do it logisically. She could've at least asked me how things were going or send me $5 to buy OUR mom a hamburger but she never did, not one time. Yes, I was right to be angry, tired and frustrated but I should've just acted like she didn't exist because that's what she wanted...not to exist in this caretaking expedition. Now that mom's gone, sis will never talk to me again. I didn't approve of her actions or absence but I still love her. My parents are gone and now my sister is gone. I have no one to share childhood memories. Yes, I have kids and grandkids but the past is gone.
I wish you the best in getting this solved, one way or another. You may not get the outcome you want but at least you can feel good that you tried.